(1) Field of the Invention
The present invention relates to resin materials for optical instruments, particularly to resin materials for optical instruments which may be suitably used for optical disks such as digital audio disks, digital video disks, and optical memory disks, various kinds of lenses, prisms, mirrors, optical fibers, photoconductive elements, and the like.
(2) Description of the Related Art
Various plastics have been proposed as the resin materials for optical instruments. The properties required of resin materials for optical instruments include heat resistance, low moisture permeability, impact resistance, good optical properties, and the like. As the resin materials satisfying these properties required, for instance, polymethylmethacrylate or polycarbonates made from 2,2-bis(4-hydroxyphenyl)propane have been used. The former however is insufficient in the properties of heat resistance, moisture resistance, and impact resistance. On the other hand, the latter excels in heat-resistance, moisture resistance, impact resistance, etc. but nevertheless have a problem of the large optical distortion thereof causing large birefringence. Large birefringence of a resin material will cause problems that when the resin material is formed into a plate or a sheet and then used as optical disks, the sensitivity of reading the information recorded in the disks will be decreased, or errors in information writing will occur. Particularly, in case of disks wherein optical information is recorded by means of digital signals, for example digital audio disks, digital video disks, and optical memory disks for information read and write, it is required that the optical distortion of the molded products is 20 nm or less in the phase difference. Further, in case of these optical disks, light beam penetrates with narrowing down to the recording film in the disk substrate, and light beam therefore advances obliquely in the substrate. At the time, oblique-incident light beam exhibits a particularly large phase difference and thereby aggravates the occurrence of errors in information read and write.
In Japanese Patent application Laid-open No. 163,007/1985 are disclosed polycarbonates having reduced birefringeces. The polycarbonates however are hardly suitable as materials for optical disks, since even the polycarbonates used in the working examples have still large oblique-incident light birefringent indices. Besides, the specification includes no concrete disclosure suggesting the resin materials for optical instruments of the present invention.